In semiconductor manufacturing, doping is the process of intentionally introducing impurities into a semiconductor material to change its electrical properties. The electrical performance of doped semiconductor devices will change depending on the doping density and profile. There are therefore a number of techniques to try to determine the dopant concentration in a solid consisting mainly of semiconductor material.
One class of techniques employs principles of atomic force microscopy (AFM). An atomic force microscope consists of a microscale cantilever with a sharp conductive tip (probe) at its end that is used to scan a specimen surface. The cantilever is typically silicon or silicon nitride with a tip that is covered with a conductive material and which typically has a radius of curvature on the order of nanometers.
One technique for detecting doping is scanning capacitance microscopy (SCM). Scanning capacitance microscopy (SCM) is a type of scanning probe microscopy in which a sharp probe electrode is held near or on the surface of a sample and scanned across the sample. SCM characterizes the surface of the sample using information obtained from the change in differential capacitance between the surface and the probe. More precisely SCM uses an ultra-sharp conducting probe (often Pt/Ir or Co/Cr metal covering an etched silicon probe) to form a metal-insulator-semiconductor (MIS/MOS) capacitor with a semiconductor sample if an oxide is present. When no oxide is present, a Schottky contact/capacitor is formed. When the probe and surface are in contact, an AC bias is applied, generating capacitance variations in the sample which can be detected using a GHz resonant capacitance sensor or other means. The tip is then scanned across the semiconductor's surface in 2D while the tip's height is controlled by conventional contact force feedback.
By applying an alternating bias to the metal-coated probe, carriers alternately accumulate and deplete within the semiconductor's surface, changing the tip-sample capacitance. The magnitude of this change in capacitance with the applied voltage gives information about the concentration of carriers (SCM amplitude data), whereas the difference in sign of the capacitance change relative to the applied, alternating bias carries information about the sign of the charge carriers. Because SCM functions even through an insulating layer, a finite conductivity is not required to measure the electrical properties.
When the SCM tip is brought into close proximity with the sample surface a Metal/Oxide/Semiconductor (MOS) capacitor is formed between them, where: M is the metal probe, S is the semiconductor material and O is a thin dielectric formed on the semiconductor surface. Free carriers within the sample are able to move under the influence of an AC electric field applied by the conductive probe (tip). The capacitance measured by the SCM sensor varies as the carriers move towards (accumulation) and away from (depletion) the probe. When the sample is fully depleted the measured capacitance is that of the oxide plus the depletion layer. When carriers are accumulated at the surface, the measured capacitance is that of the oxide layer. This capacitance variation in response to the tip-applied field forms the basis of the SCM measurement. Movement of free carriers and hence the amplitude of the capacitance variation is a function of the dopant level of the sample directly beneath the probe. For heavily doped materials the carriers do not move far. Hence, the measured capacitance variation between accumulation and depletion is small. The opposite is true for lightly doped semiconductors which yield a large capacitance change.
However in general these techniques can only provide indications of the relative dopant concentrations in a device, but they cannot measure the absolute dopant densities in semiconductor devices, particularly not in small regions when the dopant density varies over such regions of a wafer.
What is needed, therefore, are new methods for determining absolute dopant densities in semiconductor devices.